1. Field of the Invention
This invention relates to an electroluminescent display device, especially to an electroluminescent display device with a storage capacitance element for holding a video signal supplied to a gate of a driving transistor.
2. Description of the Related Art
An electroluminescent (referred to as EL hereinafter) display device with an EL element has been gathering attention as a display device substituting a CRT or an LCD. The development effort for the EL display device with a thin film transistor (referred to as TFT hereinafter) as a switching element for driving the EL element has been made accordingly.
FIG. 4 is an equivalent circuit diagram of one pixel of an organic EL display device. A gate signal line 51 for supplying a gate signal Gn and a drain signal line 52 for supplying a drain signal, a video signal Dm, cross each other.
An organic EL element 60, a driving TFT 40 for driving the organic EL element 60, and a pixel selecting TFT 30 for selecting the pixel are disposed near the crossing of the two signal lines. The TFT 40 is P-channel type and the TFT 30 is N-channel type.
A drain 43d of the organic El element driving TFT 40 is provided with a plus source voltage PVdd. A source 43s of the TFT 40 is connected to an anode 61 of the organic EL element 60.
The gate signal line 51 is connected to a gate 31 of the pixel selecting TFT 30 and provided with the gate signal Gn. The drain signal line 52 is connected to a drain 33d of the pixel selecting TFT 30 and provided with the video signal Dm. A source 33s of the TFT 30 is connected to a gate 41 of the TFT 40. The gate signal Gn is outputted from a gate driver circuit not shown in the figure, and the video signal Dm is outputted from a drain driver circuit not shown in the figure.
Also, the organic EL element 60 includes the anode 61, a cathode 65, and an emissive layer 63 inserted between the anode 61 and the cathode 65. The cathode 65 is provided with a minus source voltage CV.
A storage capacitance element 130 is connected to the gate 41 of the TFT 40. That is, one of the electrodes of the storage capacitance element 130 is connected to the gate 41, and the other electrode is connected to a storage capacitance electrode 131. The storage capacitance element 130 is disposed in order to hold the video signal Dm of the display pixel for one field period by keeping the electric charge corresponding to the video signal Dm.
The operation of the EL display device with the above configuration is as follows. The TFT 30 turns on when the gate signal Gn becomes high level for one horizontal period. Then, the video signal Dm is supplied from the drain signal line 52 to the gate 41 of the TFT 40 through the TFT 30. The conductance of the TFT 40 changes according to the video signal Dm supplied to the gate 41 and the corresponding driving electric current is applied to the organic EL element 60 through the TFT 40. Thus, the organic EL element 60 emits light.
FIG. 5 shows a cross-sectional view of the storage capacitance element 130 mentioned above. The TFT 30 is formed on an insulating substrate 10. The TFT 30 has the source 33s, the drain 33d, and the gate 31 formed on a gate insulating film 12. The storage capacitance electrode 131 is formed on the source 33s of the TFT 30 through the gate insulating film 12. The storage capacitance electrode 131 is provided with a predetermined stable voltage.
The storage capacitance element 130 is disposed for each of the pixels, in the conventional organic El display device, in order to maintain the voltage applied to the gate of the driving transistor for controlling the quantity of the electric current, which determines the luminescence of the organic El element 60.
When the voltage of the video signal Dm supplied to the pixel drops by a large quantity, it will affect the quality of the display. Therefore, a large capacitance value of the storage capacitance element 130 is required. That is, the area of the storage capacitance element 130 should be large.
There are a top emission type and a bottom emission type among the organic EL display devices. The light emitted from the organic EL element 60 radiates from the side of the organic EL element 60 opposite from the insulting substrate 10 in the top emission type display device. That is, the light radiates from the upper surface of the panel. On the other hand, the light emitted from the organic EL element 60 radiates from the side of the insulting substrate 10 in the bottom emission type display device.
When the area of the storage capacitance element 130 is large, it does not create any problem in the top emission type organic EL display device. However, the portion where the storage capacitance element is formed works as a blind in the bottom emission type display device, leading to decreased open aperture. In this configuration, the electric current supplied to the organic El element should be increased in order to acquire the necessary luminescence, compared to the case where the device has an enough open aperture. As a result, the life span of the organic EL element is shortened.